An Early Triassic Ichthyopterygian From Japan

An Early Triassic ichthyopterygian fossil from the Osawa Formation in Minamisanriku Town, Miyagi Prefecture, Japan

Authors:

TAKAHASHI et al

Abstract:

The Ichthyopterygia Owen 1840 is a remarkable group of Mesozoic marine reptiles with distinctive fish-shaped bodies. There are conflicting views on the taxonomic nomenclature of ichthyopterygians (e.g., Motani and McGowan, 2003; Maisch, 2010). In this study, we employed the phylogeny and nomenclature of McGowan and Motani (2003) in which the term Ichthyosauria refers to a derived clade within the Ichthyopterygia; according to this system, Utatsusaurus is a basal ichthyopterygian and not an ichthyosaurian. The phylogenetic relationships and systematics within the group have been analyzed in recent synthetic studies (e.g., Motani, 1999; Sander, 2000; McGowan and Motani, 2003; Maisch 2010), but a consensus is yet to be reached regarding the phylogenetic relationship with other reptiles (e.g., McGowan and Motani, 2003). More information on early ichthyopterygians is crucial to understand their origin. Ichthyopterygian fossil record extends from the Early Triassic Olenekian to the Late Cretaceous Cenomanian (Motani and McGowan, 2003). Early Triassic taxa have been reported from Europe, North America, and Asia (Table 1). Note that some species are represented by fragmentary specimens, whereas the ichthyopterygian affinity of a few taxa have been debated (e.g., Omphalosaurus and “Pessopteryx nisseri”; Motani, 2000, Sander and Faber 2003; McGowan and Motani, 2003; Dalla Vecchia, 2004; Maisch, 2010).

Permian–Triassic mass extinction affected the marine ecosystem seriously (Raup, 1979; Sepkoski, 1984), and the Paleozoic fauna was largely replaced by the modern fauna (Sepkoski, 1984). The occurrence of marine reptiles added a new trophic level of top predators in the Mesozoic marine ecosystem, and their radiation started in the Olenekian (Chen and Benton, 2012; Fröbisch et al., 2013). However, only a limited number of Early Triassic reptilian faunas actually document this process (e.g., Sulphur Mountain Formation in Canada, Vikinghøgda Formation in Norway, Nanlinghu Formation in China: see Table 1). The Osawa reptilian fauna is important in this regard as well.

In 2007, a partial skeleton of a reptile was discovered from the Lower Triassic Osawa Formation (Spathian or late Olenekian: Bando and Shimoyama, 1974; Bando and Ehiro, 1982), Inai Group, in Miyagi Prefecture, northeastern Japan (Fig. 1). This formation has yielded a number of articulated skeletons of the primitive ichthyopterygian Utatsusaurus hataii Shikama et al. 1978, which is one of the oldest animals secondarily adapted to life in open (Nakajima et al., in press). Apart from U. hataii, only two other vertebrate taxa, i.e., hybodontoid shark (Kato et al., 1995) and “Metanothosaurus nipponicus” Yabe and Shikama 1948, have been reported from the formation to science. The holotype and only specimen of the latter taxon has been missing, and the existing information is not sufficient to determine its taxonomic status (e.g., sauropterygian in Mazin [1986]; ichthyosaur and not sauropterygian in Rieppel [2000: 109]); hence this taxon is regarded as nomen dubium.

The new specimen was discovered at the base of the dark gray shale exposed along the coast in Minamisanriku Town in the northeastern corner of the Miyagi Prefecture. It was collected as a float but retained fresh surface. The locality is within 1 km from the holotype locality of Utatsusaurus hataii, and the horizon of the new specimen is correlated to the middle part of the formation and is within the stratigraphic range of Utatsusaurus (Shikama et al., 1978).